Transceivers for radio communication that use the same antenna for transmission and for reception of radio signals often require a diplexer, or duplex filter. This is particularly true when the transceiver uses a frequency-division duplex scheme, i.e. the transmit and receive bands are at different frequencies.
Duplex filters are often large and expensive because of their high performance requirements. For antenna arrays having several individual antenna elements and a corresponding number of transceivers, the cost and space requirements for the duplex filters are significant. A typical antenna array may require 16 duplex filters, the performance requirements of which are similar to that required of the filter in a full-power remote radio head (for example) with the main exception of its power handling. The realization of a duplex filter, for an acceptable size and cost, has the potential of making the flexibility of active antenna arrays available at competitive prices.
Especially in the case of handsets, a different approach has become popular over the last years: vector cancellation has been proposed for eliminating the diplexer, for example in chapter 4 of “RF and Baseband Techniques for Software Defined Radio” by Peter Kenington, Artech House, 2005, ISBN: 1-58053-793-6. Vector cancellation has also been proposed for interference cancellation (again, largely within handsets)—for example, removal of GPS local oscillator interference from a cellular handset receive chain (in a handset with GPS capability). Vector cancellation has also been proposed for Tx/Rx isolation enhancement in repeater system.
The international patent application WO 2007/013900 A2, entitled “Method and system for automatic control in an interference cancellation device” proposes a signal processing circuit that can generate an interference cancellation signal that, when applied to the victim communications channel, cancels the detrimental interference. The signal processing circuit can dynamically adjust or update two or more aspects of the interference cancellation signal, such as an amplitude or gain parameter and a phase or delay parameter. A filter in the signal processing circuit models the channel coupling. However, the behaviour of the filter is not similar or identical to that of the receive portion of the duplex filter.